Method for washing preferentially cellulosic pulp

ABSTRACT

Washing pulp is effected in apparatus wherein the washing liquid is forced through a perforate wall into and through a body of the pulp and into a collecting space for the washing liquid. To prevent clogging of the perforate wall, the influx of treatment liquid into the collecting space is interrupted at suitable intervals, the interruption being produced by pressure variations induced by a vibrator operating in the liquid.

United States Patent Inventor Nils Gustav Leffler Sundsvall, Sweden Appl. No. 720,717 Filed Apr. 11, 1968 Patented Nov. 23, 1971 Assignee Sunds Aktiebolag Saudsvall, Sweden Priority Apr. 13, 1967 Sweden 5183/67 METHOD FOR WASHING PREFERENTIALLY CELLULOSIC PULP 8 Claims, 4 Drawing Figs.

U.S. Cl 8/156,

8/137,68/181. 162/60 int. Cl D2lc 9/00 Field of Search 8/137, 156;

[56] References Cited UNITED STATES PATENTS 2,683,366 7/1954 Kuluik et a1. 8/156 UX 2,712,488 7/1955 Brax et al 8/156 2,964,047 12/1960 Jackson et al. 68/181 X 3.053969 9/1962 Leffler 68/181 3,419,466 12/1968 Sepall 8/156 X Primary Examiner-Mayer Weinblatt Attorney-Pierce, Schefi'ler and Parker METHOD FOR WASHING PREFERENTIALLY CELLULOSIC PULP The present invention relates to the liquid treatment of, suspensions of fibrous materials such as cellulosic pulp and comprises both a method and a suitable apparatus for carrying out said method.

In the manufacture of bleached cellulosic pulp great advantages are obtained if, after one bleaching step, the pulp can be subjected to washing immediately in the bleaching tower instead of being washed on filters. A considerable advantage of this kind consists in that, after bleaching at about -15 percent pulp consistency, dilution of the pulp to about I percent consistency and a subsequent thickening of the pulp to about 10-15 percent consistency for the next bleaching step are avoided. One difi'iculty, both in connection with the washing of cellulosic pulp in bleaching towers and in connection with other treatments of materials, in which liquid is caused to flow between chambers located below the level of material, has turned out to reside therein that, in particular, the sieve surfaces of the collecting chambers become gradually choked up during the withdrawal of liquor from the fibrous material (the fibers) that has to take place through said surfaces.

In order to prevent clogging of the sieve surfaces they have been made rotatable and for the purpose of cleansing sieve surfaces that have already become choked up, such surfaces have been adapted to be moved axially within the sieve chambers (Swedish Pat. No. 198,496). However, the sieve chambers are both large and heavythe sieve elements usually have a diameter of 4-5 meters-and due to this it would mean a great advantage if said elements could be made stationary and the possibility of keeping the sieve surfaces clean could yet be maintained.

By means of the present invention it has been possible to realize this desideratum. The invention includes both a special process and a special arrangement, by which it is possible to keep stationary sieve surfaces free from clogging matter.

The new process is meant for the treatment, by means of a liquid, such as water for washing (but also in connection with the displacement of liquor and dewatering) of a fibrous material suspended in aliquid and being fed through a preferentially vertical and preferably cylindrical container in the axial direction thereof, said treatment being carried out in that part of the container which is filled with the material, the treatment liquid being caused to flow through the suspension of material from at least one means of supply for the treatment liquid towards at least one collecting chamber for withdrawn liquid, said chamber having a foraminous, preferably perforate wall. The new process is characterized substantially in that the influx of the withdrawn liquid into the collecting chamber, is, entirely or partly, interrupted at suitable intervals in order to prevent clogging of the perforate wall of the collecting chamber and/or to facilitate the movement of the material past or along said surface. It is thought that this effect will be obtained thereby that the pulsatory flow will, inter alia, prevent fibers from getting stuck in the sieve holes, thus increasing the frictional resistance, and/or creating a film of liquid on the sieve surfaces in order to make it easier for the cellulosic pulp to slide past the same. Such film of liquid may also be obtained by spraying liquid along that sieve surface which is turned against the pulp and causing such spraying to take place in the same direction as the direction of feed of the pulp from one or more slots, located one after the other in the axial direction of the container all the way around along the sieve surface. An effective and correctly adjusted rinsing may result therein that the liquid in the collecting chamber inside the sieve plate member need not be pulsated, in which case the members serving that purpose may be dispensed with. The pressure variations are obtained through the intermediary of liquid, which by means of a vibrator or some other means, producing pulsations in the liquid, is pulsated within at least one conduit communicating with the collecting chamber. The pulsation of the liquid within the collecting chamber may be caused to take place either continuously or intermittently.

Also, to the liquid in the collecting chamber there may be imparted, at suitable intervals, one or more pressure surges and/or slower pressure rises. When performing the reciprocating flow the liquid should have a resultant flow into the collecting chamber for withdrawn liquid (the sieve device).

An apparatus for carrying out the new method comprises a, preferentially, vertical and preferably cylindrical container, adapted for axial feed therethrough of the material to be treated, at least one means of supply for treatment liquid located within said container and below the intended level of material therein and furthennore at least one collecting chamber for withdrawn liquid arranged in a corresponding way but displaced in the radial direction of the container. The apparatus is substantially characterized in that to the collecting chambers for withdrawn liquid there is connected at least one member for transmitting pressure impulses to the liquid in said collecting chambers from means serving for generating pressure variations.

These and other characteristics of the invention will appear from the following detailed description of a preferred embodiment, reference being had therein to the accompanying diagrammatical drawing, in which FIG. 1 is a vertical sectional view of part of the apparatus, when mounted in a vertical container for the treatment of fibrous material, FIG. 2, on a larger scale, illustrating a longitudinal sectional view of the means for pulsating the treatment liquid. FIG. 3 is a sectional view on the line IIIIII of FIG. 1 and FIG. 4, finally, a vertical sectional view of the collecting chamber.

Reference numeral 1 indicates part of the upper portion of a vertical, preferably cylindrical container, arranged to be traversed continuously by material to be treated, preferentially cellulosic pulp 2, in the direction indicated by capital A arrows. The treatment (washing) liquid is indicated by 3, and 4 is tower scraping means, of a known kind, rigidly mounted on a rotary shaft 5. 6, 7 and 8 are supply means for washing liquid and said means may be stationarily and/or rotatably mounted. Also, they may be embodied in a known manner, for instance as substantially vertically extending spray pipes or nozzles, which are movable along paths concentric with annular collecting chambers for withdrawn liquid in accordance with the disclosure of Swedish Pat. Specification 198,496. 9 and 10 indicate collecting chambers for withdrawn liquid, said chambers preferably being devised as sieve baskets. One or more pipes 11 communicate with the collecting chambers 9' and 10, said pipes serving for leading away the withdrawn liquid by way of an overflow 12. H indicates the height of the static pressure determining the speed of How of the liquid through the pipe 11. Also one (or more) pipe 13 communicates with the collecting chambers 9 and 10, said pipe 13 being, at its one end, by way of a funnellike portion 14 connected to means 15 for generating pressure variations (a vibrator or a pulsator), which through the intermediary of that liquid which normally is to be present in the pipe 13 and in the funnellike portion 14, are to produce a pulsatory flow of liquid through the slots or openings 16 in the walls of the chambers 9 and 10. In the embodiment shown the vibrator 15 comprises two centrifugal weights 18, adapted to rotate in opposite directions via gear drives 17. The vibrator is adapted to actuate a membrane 20, clamped between flanges 19, said membrane, by way of the liquid in the pipe l3, l4, transmitting the movements emanating from the vibrator, so that the previously mentioned, pulsatory flow through the slots 16 is obtained. The pipe 13 directs the pulsatory flow towards the sieve baskets 9, l0 and not towards the outlet pipe 11. Instead of the vibrator 15 there may be used some other means or some other member for generating the pulsatory flow of liquid through the slots or openings 16. Such other means may consist of, for instance, a piston reciprocating in a housing. The piston may be driven continuously or intermittently and its movements may be rapid or slow, even very slow. The use of a piston has proved particularly suitable for slow movements and long lengths of stroke or in case a rapid pressure surge is desired for the purpose of cleansing the slots or holes during the movement of the piston in one direction and in case a uniform and smooth withdrawal is desired during the return of the piston in the opposite direction. Consequently, in connection with this last-mentioned functioning the piston is to move rapidly in one direction and then to return slowly in the other direction. The liquid within the housing of the piston may either be in a direct, open connection with the collecting chambers 9, or in a power transmitting connection by way of a membrane or the like. After one single stroke, i.e. a movement in one direction, the piston may remain immobile in the end position for a certain period of time, alternatively its resting position may be occupied after one or more double strokes. 21 and 22 indicate supply conduits for washing liquid to the chamber 8 and, respectively, to the chambers 6 and 7. By way of control valves (not shown) said supply conduits are connected to a water conduit or to some other conduit, possibly also to a backwater conduit, i.e. in such a manner that the amount of liquid and the pressure may be controlled. As shown, the conduit 2] ends above the arms of the tower scraping means 4. Treated material (washed pulp) leaves through the outlet 23.

In order that the material, for instance cellulosic pulp, shall be moved in the direction of the arrow A, FIG. 4 (i.e. upwards in the embodiment shown) it is required that the pressure p, be greater than the sum of the pressure p, and the difference in static pressure between the levels p, and p, at friction-free flow. If the difierence between these pressures is great the flow B of the displacement liquid may give larger inflow volumes at the lower portions of the sieve baskets (the collecting chambers) 9, 10 than at the upper portions.

According to the embodiment, shown in FIG. 3, the collecting chamber 9 is meant to have four outlet pipes, uniformly distributed along the circumference. All these pipes (with pipes between the collecting chambers) are indicated by Ill. The collecting chamber 10 is meant to have two connections to pipes 11, counting along its circumference. The number of collecting pipes may be larger or smaller. A collecting chamber having a large diameter should have more pipes 11 than a chamber having a smaller diameter. Such pipes also may extend out of the tower, completely separated from each other. Theoretically, the pitch around the collecting chambers should be equal, irrespective of the diameters of the chambers. The same thing applies to the inlet pipes of the chambers 6, 7 for supplying the washing liquid. In these chambers the liquid should not have to be given a pulsatory motion, since the flow of liquid proceeds in a direction out of the chambers and the fibers of the cellulosic pulp should not have any tendency to cause clogging of slots or holes. Despite this fact it may, under certain circumstances, be of advantage to supply the chambers with intermittent pressure surges through the liquid in the supply pipe (pipes) and, under particularly difficult conditions, with continuous pressure surges.

The invention may, with advantage, be used also in such apparatus in which cellulosic pulp is dewatered from 1-5 percent to about 8-15 percent in vertical containers with concentric sieve baskets arranged, in principle, in the way shown in FIG. 1. Such sieve baskets may be provided at the top, at the middle or at the bottom portion of the container. During the axial flow of the cellulosic pulp within the container the dewaterin g may be followed by dilution to, for instance, l-5 percent, and repeated dewatering to about 8-15 percent by means of two successive sets of sieve baskets, when counting in the direction of flow of the cellulosic pulp. Alternatively the supplying of liquid may then take place by rotatable spray pipes in a known manner in accordance with the previously mentioned Swedish Pat. No. l98,496. During the dilution and, respectively, the dewatering steps the sieve holes may be kept clean by pressure surges with clean water, white water or some other suitable liquid.

A further object of the invention is that it should be possible to move the cellulosic pulp from one container to another (for instance, from one bleaching tower to a successive bleaching tower) without other dilution takintg place than the one which might be obtained from a liqut treatment agent of the bleaching agent type, the condensate type et cetera.

The feeding of the cellulosic pulp may take place intermittently or shockwise, in which case no, or a limited, washing takes place during the feeding of the cellulosic pulp (in the direction of the arrow A).

Although unsuitable, in view of the power requirement and strength stresses, the sieve members (the collecting chambers) 9, 10 may be given a vibratory motion in the axial and/or radial direction of the container 1, by which the liquid need not be pulsated by means of the special means described above. The sieve surfaces (i.e. the foraminous parts) of the sieve members may be parallel to the direction of flow of the cellulosic pulp, as shown in FIGS. 1, 3 and 4, or they may form an angle, to said direction.

As a summary it may be emphasized that the method and/or the apparatus according to the invention is particularly suitable for the treatment-by means of a liquid-of suspensions of fibers or materials within the range of concentration of 1-15 percent.

What is claimed is:

l. A method for the continuous treatment of a suspension of a fibrous material comprising introducing the suspension into one end of a substantially vertical container, moving the suspension axially through the container, said container being, in the part where the treatment is carried out, filled with the suspension, introducing a treatment liquid into the moving suspension at a plurality of points located, with respect to each other, both axially and transversely of the container, passing the treatment liquid in a direction forming an angle with the direction of movement of the suspension, withdrawing liquid from the suspension at a plurality of points located, with respect to each other, both axially and transversely of the container, the points of withdrawal of liquid being arranged stationan'ly and spaced from the points of introduction of the treatment liquid transversely of the container, the space between the points of introduction and withdrawal being filled with the suspension of fibrous material, combining the liquid withdrawn at the plurality of points of withdrawal, passing the withdrawn liquid out of the container, periodically interrupting, entirely or partly, the flow of liquid at the points of withdrawal thereof from the suspension so that any of the fibrous material which may tend to clog the points of withdrawal is permitted to continue its passage axially through the container and removing treated suspension from the opposite end of the container.

2. A method as claimed in claim I wherein the step of periodically interrupting the flow of liquid at the points of withdrawal from the suspension comprises subjecting the liquid to pressure impulses.

3. A method as claimed in claim 1 wherein the suspension of fibrous material being subjected to treatment with the treatment liquid has a concentration of from about 1 percent to about 15 percent fibrous material.

4. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently.

5. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently and suddenly to more positively effect a removal of any fibrous material which may have tended to clog the points of withdrawal.

6. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently and at regular time intervals.

7. A method as claimed in claim 2 wherein the pressure impulses are initiated and concluded gradually.

8. A method as claimed in claim 5 and further comprising initiating the pressure impulses suddenly and concluding the impulses gradually. 

2. A method as claimed in claim 1 wherein the step of periodically interrupting the flow of liquid at the points of withdrawal from the suspension comprises subjecting the liquid to pressure impulses.
 3. A method as claimed in claim 1 wherein the suspension of fibrous material being subjected to treatment with the treatment liquid has a concentration of from about 1 percent to about 15 percent fibrous material.
 4. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently.
 5. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently and suddenly to more positively effect a removal of any fibrous material which may have tended to clog the points of withdrawal.
 6. A method as claimed in claim 2 wherein the pressure impulses are carried out intermittently and at regular time intervals.
 7. A method as claimed in claim 2 wherein the pressure impulses are initiated and concluded gradually.
 8. A method as claimed in claim 5 and further comprising initiating the pressure impulses suddenly and concluding the impulses gradually. 